1. Field of the Invention
The present invention relates to a front/rear wheel torque distribution control apparatus applied to a four-wheel drive vehicle, which variably controls a driving torque distribution to a front wheel and a rear wheel by controlling a tightening torque of an electronic control clutch.
2. Description of the Related Art
A technology has been heretofore known, in which in the case where a four-wheel drive vehicle having an electronic control clutch for controlling a torque distribution transmitted to front and rear wheels is mounted with a tire having a diameter different from that of a tire originally mounted thereon (hereinafter the tire having the diameter different from that of the tire originally mounted is referred to as a different-diameter tire, and the tire originally mounted is a normal-diameter tire), after the different-diameter tires are detected, a front/rear wheel distributed torque is not controlled (an alarm alone is raised) or the front/rear distributed torque is made close to a two-wheel drive side (see Japanese Patent Application Laid-Open No. 4(1992)-103433).
However, the conventional front/rear wheel torque distribution control apparatus of the four-wheel drive vehicle has the following problems.
When the distribution torque is large despite of an increase of a vehicle speed (a clutch tightening state), a difference of a rotation speed between the front wheel and the rear wheel, which is produced by the different-diameter tires, cannot be absorbed by a relative rotation of a plate of the electronic control clutch. A drive train may be adversely affected as long as a driver does not cope with such a situation with replacement of the tire with new one and the like.
When the torque distribution is made to be a sufficiently small constant value (two-wheel drive side), four-wheel drive performance in which a good starting characteristic is to be obtained in starting a vehicle is deteriorated.
The first object of the present invention is to provide a front/rear wheel torque distribution control apparatus of a four-wheel drive vehicle capable of reducing bad influence of different-diameter tires on a drive train without damaging inherent four-wheel drive performance enhancing a starting property at the time of a start of the vehicle at which large front/rear wheel distribution torque is required.
The second object of the present invention is to provide a front/rear wheel torque distribution control apparatus of a four-wheel vehicle capable of securing the lowest four-wheel drive performance in a normal running state of a vehicle by allowing a front/rear wheel distribution torque to approach to a torque at the time normal tires are mounted thereon.
To achieve the first object of the present invention, a first aspect of the present invention provides a front/rear wheel torque distribution control apparatus of a four-wheel drive vehicle having an electronic control clutch for controlling a torque distribution to front and rear wheels, in which one of the front and rear wheels is a main drive wheel and the other is a subsidiary drive wheel,
the front/rear wheel torque distribution control apparatus comprising:
a tire diameter difference degree detector for detecting a degree of a diameter difference between a different-diameter tire and a normal-diameter tire (hereinafter referred to as a degree of a diameter difference) when any of the front and rear wheels mounts the different-diameter tires;
a driver request torque calculator for calculating a driver request torque transmitted from the main drive wheel to the subsidiary drive wheel via the electronic control clutch in response to an operation of a driver;
a vehicle speed detector for detecting a vehicle speed;
a starting time limit speed determiner for making a starting time limit speed in which a torque at the time of starting the vehicle is limited by a vehicle speed lower as the degree of the tire diameter difference detected by the tire diameter difference degree detector is larger; and
a starting time torque distribution controller for outputting an instruction to obtain the driver request torque calculated by the driver request torque calculator to the electronic control clutch until the vehicle speed becomes equal to a starting time limit vehicle speed determined by the starting time limit speed determiner when the vehicle mounts the different-diameter tires.
In the first aspect of the present invention, the tire diameter difference degree detector detects the degree of the diameter difference when the vehicle mounts the different-diameter tires on any of the front and rear wheels. In response to the operation of the driver, the driver request torque calculator calculates the driver request torque transmitted via the electronic control clutch. The vehicle speed detector detects the vehicle speed.
When the detected degree of the tire diameter difference is higher than a setting value, the starting time limit speed determiner determines the limit torque so as to be smaller as the degree of the diameter difference is lager.
When the vehicle mounted with the different-diameter tires starts, the starting time torque distribution controller outputs an instruction to obtain the driver request torque calculated by the driver request torque calculator to the electronic control clutch until the detected vehicle speed becomes equal to the starting time limit vehicle speed. The torque distribution transmitted to the front and rear wheels is control to a four-wheel drive side.
Specifically, at the time of starting of the vehicle when the front/rear wheel rotation speed difference occurs due to a large drive slip, a distribution of the drive torque to the four wheels as evenly as possible and acquisition of a starting property suppressing the drive slip are wished. However, when a torque distribution control is performed for the vehicle mounted with the different-diameter tire just in the same manner for the vehicle mounted with the normal-diameter tire and a perfect clutch tightening state is maintained because of a large front/rear wheel distribution torque in spite of an increased vehicle speed, the front/rear wheel rotation speed difference caused by the different-diameter tire cannot be absorbed by the electronic control clutch, and a twist torque occurs in a drive train (drive system). Furthermore, when a clutch tightening force is not sufficiently high, the front/rear wheel rotation speed difference caused by the different-diameter tire can be absorbed by a relative rotation of a clutch plate. However, a high friction heat is generated because of the relative rotation in the state where the clutch tightening force is applied, so that deterioration of the clutch due to heat is promoted. In any case, if a driver does not cope with such a situation by some countermeasures such as replacement of the tire with new one, a drive train including the electronic control clutch is adversely affected.
On the other hand, when the torque distribution to the front and rear wheels is made to be close to a two-wheel drive side in starting of the vehicle mounted with the different-diameter tires, the adverse influence on the drive train is removed. However, a high starting property utilizing a four-wheel drive performance cannot be obtained at the time of starting of the vehicle.
Accordingly, a vehicle speed (starting time limit vehicle speed) which preferentially permits the torque distribution to the front and rear wheels at the time of starting of the vehicle is switched in accordance with the degree of the tire diameter difference.
Thus, under the restricted condition that the driver request torque is output until the vehicle speed becomes equal to the starting time limit vehicle speed, the four-wheel drive performance can be displayed by the driver request torque.
Accordingly, at the time of starting of the vehicle where a large front/rear wheel distribution torque is necessary, it is possible to reduce the adverse influence on the drive train owing to the different-diameter tire without damaging the inherent four-wheel drive performance enhancing the starting property.
To achieve the second object of the present invention, a second aspect of the present invention provides a front/rear wheel torque distribution control apparatus of a four-wheel drive vehicle having an electronic control clutch for controlling a torque distribution to front and rear wheels,
the front/rear wheel torque distribution control apparatus comprising:
a tire diameter difference degree detector for detecting a degree of a diameter difference when any of front and rear wheels mounts different-diameter tires;
a normal running time detector for detecting a normal running time of the vehicle;
a normal running torque gain determiner for determining a normal running torque gain to be a torque gain smaller than a torque gain at the time when normal-diameter tires are mounted as the degree of the diameter difference detected by the tire diameter difference degree detector is larger, when any of the front and rear wheels mounts the different-diameter tires;
a normal running torque calculator for calculating a normal running torque by multiplying a front/rear wheel rotation speed difference by the torque gain determined by the normal running torque gain determiner; and
a normal running time torque distribution controller for outputting an instruction to obtain the normal running torque calculated by the normal running torque calculator to the electronic control clutch, when the vehicle makes normal running with the different-diameter tires mounted on any of the front and rear wheels thereof.
In the second aspect of the present invention, the tire diameter difference degree detector detects the degree of the diameter difference when any of the front and rear wheels mounts the different-diameter tires. The normal running time detector detects the normal running time of the vehicle. The normal running torque gain determiner determines the normal running torque gain to be the torque gain smaller than the torque gain at the time when the normal-diameter tires are mounted as the degree of the diameter difference detected by the tire diameter difference degree detector is larger, when any of the front and rear wheels mounts the different-diameter tires. The normal running torque calculator calculates the normal running torque by multiplying the front/rear wheel rotation speed difference by the torque gain determined by the normal running torque gain determiner.
The normal running time torque distribution controller outputs the instruction to obtain the calculated normal running torque to the electronic control clutch, when the vehicle makes the normal running with the different-diameter tires mounted on any of the front and rear wheels thereof, and the torque distribution transmitted to the front and rear wheels is controlled.
Specifically, when the torque distribution control corresponding to the front/rear wheel rotation speed difference is performed for the vehicle mounted with the different-diameter tire just in the same manner for the vehicle mounted with the normal-diameter tire, the front/rear wheel distribution torque becomes larger in accordance with an amount of the front/rear wheel rotation speed difference caused due to the different-diameter tire.
Accordingly, when the vehicle makes the normal running with the different-diameter tires mounted thereon, the front/rear wheel distribution torque caused by the front/rear wheel rotation speed difference due to the different-diameter tire is suppressed by switching the torque gain in accordance with the degree of the diameter difference.
With such an operation, by switching the torque gain, it is possible to remove or reduce an influence of an increase in the front/rear wheel distribution torque owing to the different-diameter tire, and hence the front/rear wheel distribution torque becomes close to a torque of a vehicle mounted with normal-diameter tires.
Thus, in the normal running of the vehicle mounted with the different-diameter tires, the minimum four-wheel drive performance can be secured by allowing the front/rear wheel distribution torque to be close to a torque at the time the vehicle mounts the normal-diameter tires.